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Single-Phase 3-Wire –– 120/240V or 120/208V Metering

Modified on: Thu, 17 Aug, 2017 at 9:35 AM

Single phase 3-wire systems (some call them two phase or Edison) have 2 hot wires and a neutral wire. This is the most common residential system in the USA. If you measure from hot wire to hot wire you will get 240 volts and if you measure from either hot wire to neutral you will get 120 volts. Metering of these systems can be done with any of our Omnimeter models. Metering 120/240 volt 3-wire systems can also be accomplished using our EKM-25IDS, but the instructions below will focus on the EKM Omnimeters.

The images and diagrams shown here are in no way exhaustive. These are just a few examples of how you might set up a metering system, and are simply intend to show what is possible. These are the options that you might consider for your metering setup:

Do I want to read the meter(s) locally with computer or do I want a cloud based solution (EKM Push).

What diameter and amperage rating will I need for the Current Transformers? You will need two CTs for your 3-wire single phase electrical system.

Do I need to go wireless?

Do I have a need to daisy chain multiple Omnimeters together? This allows you to connect multiple Omnimeters in series, which can then be connected to a single communication device.

The answers to these questions should help you narrow down your options and allow you to select the products that you will need for your particular application.

Omnimeter Connection Instructions:

For a 120/240 volt 3-wire system you will need to use 2 CTs. You will have 2 hot wires (usually black or red) and a neutral wire (usually white). When you measure the voltage between the 2 hot wires you will get 240 volts, when you measure from either hot to neutral you will get 120 volts.

Turn off power to the subpanel.

Mark one of the hot wires as L1 and mark the other hot line as L2. You can pick either hot to be L1.

Connect the L1 hot line voltage reference wire to port 7 on the Omnimeter, connect the L2 hot line voltage reference wire to port 8 on the Omnimeter and connect the Neutral line voltage reference wire to port 10 on the Omnimeter.

Connect the CT1 so that the L1 hot wire passes though it and the arrow on the CT points towards the load. Connect the CT2 so that the L2 hot wire passes though it and the arrow on the CT points towards the load.

Connect the CT1 black wire will go to port 1 on the Omnimeter and the CT1 white wire will go to port 2 on the Omnimeter. Connect the CT2 black wire will go to port 3 on the Omnimeter and the CT2 white wire will go to port 4 on the Omnimeter.

This video tutorial provide an example of how to install an Omnimeter Pulse v.4 on a 120/240v electrical system:

See this model for reference of how to properly connect an Omnimeter to a 120/240 volt 3-wire system:

Please note that most of the diagrams above show a wireless connection using 485Bee Zigbee radios. The 485Bee radios are an option that should only be considered if a hardwire connection is not possible, as they will add significant cost and complexity.